The present invention relates to ring lasers, and, more particularly, to solid state ring lasers.
Optical communications systems require lasers which are compatible with optical fibers and provide stable single-mode transmissions sufficiently narrow for high-baud rate heterodyne detection systems. To be commercially viable, the laser sources should have low power requirements, high reliability, modest size and reasonable cost.
Typical optic fibers transmit wavelengths of about 1.3 microns (.mu.m) most efficiently. To accommodate a target rate of 100 megabaud of phase-shift keyed data, for example, requires a single-mode line width less than 200 kilohertz (kHz). The commercial requirements point to a solid-state system with a pump threshold low enough to be met by semiconductor lasers.
While seemingly meeting the commercial requirements for optical communications systems, conventional semiconductor lasers tend to run multimode, and thus are not well-suited to heterodyne detection methods, such as phase-shift keying. Some systems, e.g., those using distributed feedback, have achieved single-mode operation of semiconductor lasers. However, the line widths involved are on the order of 20 megahertz (MHz), much too broad for the targeted data rates.
A particularly promising class of solid-state lasers are based on crystals doped with a laser material, for example, yttrium aluminum garnet (YAG) crystal doped with neodymium, the combination being referred to as "Nd:YAG". As a laser medium, neodymium's output range includes wavelengths well-suited for optic fibers, e.g., 1.319 .mu.m and 1.338 .mu.m, and some secondarily acceptable wavelengths, such as 1.062 .mu.m.
While neodymium can be used with several alternative crytal host materials, Nd:YAG has the advantage of a relatively low pump threshold. The low pump threshold is important to accommodate semiconductor lasers as pumping sources for the neodymium-doped crystal.
Solid-state laser media, such as Nd:YAG, are not well-suited for single-mode standing-wave oscillation since standing waves can deplete the gain in a spatially inhomogeneous way, inducing extraneous modes. By using the doped crystals in a unidirectional ring laser arrangement, the mode-shifting behavior can be minimized since the laser modes are traveling waves which deplete the gain more evenly.
Discrete implementations of ring lasers can include a laser medium, a polarizer, a half-wave plate and a Faraday rotator. Light emitted by the laser medium and polarized by the polarizer is rotated by the half-wave plate in a direction dependent on the direction of light travel about the ring. In contrast, the direction of polarization rotation induced by the Faraday rotator is independent of travel direction.
By way of example, assume both the half-wave plate and the Faraday rotator induce 10.degree. polarization rotations. Light traveling clockwise about the ring would be subjected to two 10.degree. rotations in the same direction, so that the total rotation is 20.degree.. In contrast, light traveling in the counterclockwise direction would be subject to the opposing rotations, so the net rotation is 0.degree..
Consider light generated by a laser source and linearly polarized by a polarizer. If propagated in a ring through the Faraday rotator and the half-wave plate in a direction in which their effects cancel, the light can return to the polarizer with the originally imparted polarization. Hence, the returned light can proceed on its next trip around the ring with little loss at the polarizer. Light traveling in the opposite direction around the ring reaches the polarizer with a new polarization, so energy is absorbed by the polarizer as it repolarizes the beam. Lasing favors the direction of lower loss, which dominates to establish the unidirectional character of the laser.
Thus, Nd-doped crystal lasers including external polarizers and Faraday rotators arranged in a unidirectional ring configuration have provided narrow-band single-mode output suitable for optic fiber communications. A disadvantage of these discrete implementations is that they are very vulnerable to vibrations. Vibrations can cause frequency shifts in the laser output, which contributes to noise and interference upon detection. The discrete lasers are also disadvantageous due to bulk, expense, alignment tolerances and fragility.
The disadvantages of discrete implementations of the unidirectional ring laser are addressed by the "MISER", disclosed in U.S. Pat. No. 4,578,793 to Thomas J. Kane and Robert L. Byer. Kane and Byer also describe the MISER in "Monolithic, unidirectional single-mode Nd:YAG ring laser", OPTICS LETTERS, Vol. 10, No. 2, February, 1985.
The MISER embodies the elements required of a unidirectional ring laser in a monolithic design, which minimizes problems due to reliability and alignment tolerances that can plague discrete designs. The MISERS employs a Nd:YAG crystal as its laser medium. In the presence of an appropriately directed magnetic field, the crystal itself acts as the Faraday rotator. The out-of-plane total internal reflections (TIR) within the crystal perform a function comparable to that of the half-wave plate, and the outlet coupler at the front face acts as a partial polarizer.
In the disclosed MISER, a pump beam enters a front face of the crystal, and travels to and from the front face parallel to top and bottom faces of the crystal. However, two bevels near a rear face of the MISER crystal reflect the beams upward, causing the ring to include a point along the top face of the crystal. Thus the ring includes a large, e.g., roughly 90.degree., out-of-plane angle which induces a direction-independent polarization shift in light traveling about the ring.
The MISER disclosed in the references above requires a three kilogauss magnetic field and a 150 milliwatt (mW) pumping threshold to achieve about 5% efficiency at 1.06 .mu.m. The three kilogauss magnetic field, if supplied by permanent magnets, requires an expensive rare earth magnet. The 150 mW and about pump power is supplied by a powerful argon laser. The length of the crystal is 38 millimeters (mm) and a coating applied to the front face of the crystal established an output coupling of 1.0%.
The advantages of monolithic design could find ample use in fiber optic communications systems. However, the specifications of the disclosed MISER are not well adapted for fiber optical systems. For example, it is not commercially practicable to power complex communications networks using argon ion lasers. Semiconductor diode lasers are more reliable, compact and cost effective. Unfortunately, they cannot approach the 150 mW pump threshold of the MISER. A much smaller pump output, such as 10 mW, is a more reasonable given the limitations of current smieconductor lasers.
Another concern is that most available optic fibers transmit light much more effectively at about 1.3 .mu.m than at the 1.06 .mu.m output obtained from the disclosed MISER. The difficulty of lowering the threshold under 10 mW is compounded by the fact that the inherent gain of the about 1.3 .mu.m lines is only one-fourth that of the 1.06 .mu.m line.
MISER specifications can be altered by varying certain parameters. For example, the crystal can be made smaller and the output coupling reduced. However, any advantages obtained in lower pump threshold would be at least partially offset by the greater magnetic field required to obtain unidirectional lasing. Relatively expensive rare-earth permanent magnets are required to achieve 3 kilogauss, and larger fields would greatly increase the cost of the incorporating system.
What is needed is a ring laser which provides the advantages of monolithic design, and which can provide 1.3 .mu.m outputs with a pump threshold achievable by semiconductor diodes. In addition, the magnetic field required for Faraday rotation should be reasonable, preferably considerably below 3 kilogauss.